Combustible gas analyzing apparatus



Nov. 11, I952 Filament Resistance R. E- HARTLINE COMBUSTIBLE GAS ANALYZING APPARATUS Filed Jan. 17, 1950 C i I 4C I l l. l I A I I I I Q;

Current 1c Fig I INVENTOR. Ralph E. Hartline M I Attorney fatentecl Nov. 1 1

UNITED STATES QFFICE:

COMBUSTIBLE GAS ANALYZING. APPARATUS- RalphE; Hartline, Tulsa, Okla:, assignorto-Stano-- lind-oil and Gas Company -Tulsag Oklm; a cor poration of Delaware A plieation'Januai- 17, 1950; saama. 1353141" 4 Cl-aims. (01. 23 255) Thisinvention pertainsto"a-method and-means for analyzing a" gaseous *mixturefor combustible components: Y More" specifically; it has applica-- tion to combustible ga'sj indicators" involving aheating element, usually a filament heated to such a temperature that under certain-conditions there is-combustioii adjacent the filam'entwhich produces ananomalous heating condition atthe heating-element which-may be-detecte'd. I

It is an objectof this'inventiomto--'pr0videan improved combustible-gasindicator; It is a fur--- ther object of'this'invention to provide a portable gas indicator employing a=cyclically periodically heatedfilament which-has negligible ze'ro drift of its signaloutput under operation,- particularly 'under continuous operations" Fu'rtherobj'eets andadvantages of this invention will be apparent from this specification;

One type ofbombus-tible-gasindicatoremploys a filamentheat'e'd-by the current of an ass'ociated measuringcircuit;such asyfor-example;-a Wlieatstone bridge; The filament is heated to l a tempera'ture suifi'ci'ently high so "that under conditions combustion of any nammabi component of the sampleoccursad?aoentithe' filament surface as the same is brought into 'co'ntactwith the filamerit;- This looal combustion increases thefi1a-- ment temperaturebeyond the temperature pro"- duced by the electric currenti'and hence alters the filament resistance Such change in filament resistance 1 (usually an increa sef-is "employed toactuate a measuring circuit to givean indication Which may becalibratedir'i termsof the concentration -of the flammable component.

Such indicatorsare subject to a common difii culty, namely; that 'ch'a'nge's in ambient temperature F of the" combustible-gas sample :or in the a supply. of: voltage' feeding L the: heating of element produce variations int-the" output which may be misinterpreted as changes inf flammable co'n'cen trationof the gas: Circuit stability is commonly obtained by (adding a compensatingfilament similar to that" in rthe detector ielement to the measuring circuit as, for example", in an adjacent arm temperature or in the supply:voltageproduce ;45

of the Wheatstone/bridg'e': changes fin ambient similar: changesfin the two filaments; but their" lo cation in the 'circuitis such as to 'prh'duce' op-' positely acting aresponse-en maindicating meterw The circuit is: therefore -:some'what stabilized; Combustion on the oompen'sato'r filament istprevented by not exposing =compensatorto sample flow on by operating wat a' loweii temperature}; Instrumentsf ilofithi's ch'arac'ter are described, \foriexamplei in Sl:-'Patents- 2;2 79,39 Z and-2,244,366:

combustioii tempeeature ftr=the ga These and alrotherinuicators of the type presently infthe art are subject to 'adriit of the zero pointwhiclilunde'r some conditions of operation (particularly when detecting mixtures of air andnaimih'able gases "of high" ignition temperatures) b'ecemes 'uiteserious: Satisfactory .oper'-- ation of the heating element or detector filament inthe combustionfor the hi'gher ignition temper ature' flammable gasesj asgfor example, methane. requires; that it-operate at a relatively high temper'at'ure; L In the usual? compensated detector involving? series operation" of 'the detector and compensator 'firaments'intiie bridge circuia'a 'de-= creaseflof detector=filanientresistance with age results a decrease" in the detector" and the The introduction' of a" compensator) current. fiammableyapor' increases the detector 7 temperature'; "raising its resistance-,and thus-lowers *the Shifts ot thexzero also result fromchanges in thethermalconductivityand volumetric specific heat of: the sample when" only- :the 1' detector is expo'sedtothe sample :floiw. Partial compensation is accomplished when the compensator is deactivated and exposed mothe sample flow;

I have found-tha't theahe'ating: element of a combustible-gas --in'dicator: is iriac'tive as a combustion catalyst bel'o w' a -certain threshold temperature which c'ould also b' ca comeustien starta at' -"this rnifiini increas'esthecombustion? This" eifet is ac'cum-u lativ'e, and theprocesscontinues until-a tem'pera tiire- -increase nolo'nger results in'an-increasein combustion-a Accordingly} if the filament is operated-in acyclical 4 temperature pattern so that during a large part" of the can occur due to high led the minimum Once local combustion 1 tem erature}: the increasedheat -generated in-- creases-thefilament? temper atlireswhich in turn I e e time it operates-below the mini-mum combustion temperature 'of the-gas present; IeIa'tiVGIy it will not: be subject to" evap oration and accordingly will experience negligible zero drift. Even if the filament evaporates in part, there still will be negligible zero drift, as will appear from consideration of the detection system. This temperature variation can be achieved by supplying a cyclically varying current to the detector heating element. A varying direct current or alternating current or preferably a combination of the two in which the peak alternating current is less than the direct current can be employed. The frequency of variation of the heating is chosen low enough so that the filament thermal time lag is reasonably small as compared to the period of the variation in temperature. The filament temperature then tends to follow the variation of the square of the heating current. This heating current is chosen such that the heating-element temperature goes through an excursion from a range below the minimum combustion temperature to a temperature above this value and preferably substantially above this value. The detection circuit operates on an anomalous variation in the resistance of the heating element which occurs when local combustion of the flammable components of the gas being tested occurs at or close to the heatingelement surface. It will appear from this specication that there is what might be called a hysteresis effect in filament resistance as the filament current is changed whenever local combustion takes place. This hysteresis efiect distorts the voltage drop across the heating element and accordingly varies the harmonic content of this voltage, which in turn can be detected by suitable means.

In order to illustrate the principles involved more completely, certain figures are attached to this specification and made a part thereof. It is to be understood that these are merely by way of illustration and do not limit the scope of the invention. In these drawings:

Figure l is a diagram of the change of filament resistance with current during one cycle of operation of the detector;

Figure 2 is a circuit diagram of one complete form of the combustible-gas detector; and

Figure 3 is a view of the physical arrangement of apparatus in one such detector of the type of which the circuit diagram is given in Figure 2.

Consider in connection with Figure 1 a variation in current through a filament exposed to a combustible gas. At zero current, such a heating element will have an electrical resistance as at point A. Increase in the heating current through the resistance increases this resistance along the path E-B. If no combustible gas were present, the resistance of the heating element would increase along some path B-C'. However, if the temperature corresponding to' the filament resistance at point B is sufilcient to cause a start of local combustion at the heating element, and if a combustible gas is present, there will be an increase in the temperature of the filament in excess of that produced by the heating current, and a consequent increase of filament resistance which, as discussed above, will cause usually a further increase in combustion, so that the resistance increases to some value C. Upon lowering the filament current, the filament temperature will decrease along the path C--D, since under the flow of combustible gas there is still local combustion until the temperature decreases to such a value that the filament is below the minimum combustion temperature of the gas. This will occur at a point D on the variation of filament resistance plot, which is approximately the same resistance of the filament as at the point B at which local combustion began. The filament resistance will then drop relatively rapidly until at some point E it comes back to the original curve AE-B.

The net results could be termed a thermal or resistance hysteresis loop around the values EBCD -E as the filament current is varied over a range including a temperature corresponding to the minimum combustion temperature of the gas present. Operation over this loop will occur only in the presence of a combustible vapor, and, if the flammable content decreases to zero, operation of the heating current over values Ie to I0, corresponding to the variation from E- to C, will cause no such variation in filament resistance, and the retrace of the filament resistance against current will be along C-B-E. Accordingly, if a varying current is applied which will vary between the limits corresponding to points E (current 1e), and C or C (current Ic), a circuit measuring the abnormal variation in resistance along the path 3-0 will detect the presence of this combustible gas.

Obviously, the wave form of the applied current is not limited. The type of detector circuit employed determines the character of the cyclically varying generating means used to vary the heating element through the temperature range from below to above the minimum combustion temperature of said gas. The responsive means has the requirement on it that it produce an indication upon the heating of the heating element by local combustion of the combustible gas only.

One advantageous type of heating current which can be employed can be considered to be a combination of a constant direct current with a superimposed alternating current, preferably with a peak value less than that of the direct current. The application of this varying current to the filament with a period of the alternating current at least an appreciable fraction of and preferably large compared to the time lag of the filament produces a cyclic variation in filament resistance which in the presence of a non-flammable gas would vary between points such as E (corresponding to current Is) and 0 (corresponding to current 10). The introduction of a combustible constituent in the vicinity of the filament results in an increase in the filament resistance above point B, resulting in a marked increase of the harmonic content of the total voltage or voltage drop across the heating element. By employing a detection circuit which produces an indication which is responsive to the changes of harmonic content of the voltage drop across the heating circuit, a combustible-gas detector is produced which has negligible zero shift. The increase in harmonic content of the voltage across the heating element, it is apparent, is an increase in the even harmonics of the alternating-current component of the heat current. Any

detector which produces an output indication responsive to an increase in one or more of the even harmonics of the voltage drop across the heating element can be employed as the indicat- One convenient arrangement of circuit elements to accomplish this is shown in Figure 2.

In this figure, an alternating-current generator ll is applied through a transformer l2 and current-adjusting resistance l3 to a heating element or filament [4 in a chamber l5 through which across conductors 3|. theory of balanced modulators, if a current of local combustion of the s current from battery 'l'll 'thus producing a variat qaia.hea in .wrr atthro sh; re i erg tfiw V ami im mraluefi rr p l mam. valu rrespond is glen h maxi value producing a temperature in the heat g, lement, @PQYQ i3i P-I qqmh is patemmrature of the gas tofbe detected This can determined by passing alternately I10I1-COHl b l lt l his and combustible gases through the chamber i 5 and noting the variation in the output of this circuit.

The output circuit or detector means in the cir- 23,, a

in a well for geochemical analysisor the like, it-

cuit shown consists of a balanced modulator indicated generally by numeral and composed of resistors 21 and 22 of substantially equal value, resistors 23 and 24, also of substantially equal value, and rectifiers'25, 26, 21, and 28. This modulator is coupled, for example, by coupling condensers 29 and 30 across the heating element M and is thus responsive to variations in voltage drop across this element. Alternating current applied across condensers 29 and 30 produces novoltage drop across the outside end of resistors 23 and 2G in the absence of a biasing voltage As is well known in the frequency f is applied across conductors 3!, the peak value of which is substantially greater than that of any voltage applied from heating element M, the direct component of voltage output across the outside terminals of resistors 23 and 24 is proportional to the component of input voltage at the frequency f,

Since in this circuit the detection element is operating on an increase in even-harmonic content of the voltag drop across resistor M, a frequency 1 across the conductors 3| which is any even multiple of the frequency of generator ll could be employed. I prefer to use the second harmonic of generator H, and accordingly produce, by the full-wave rectifier 32 and band-pass filter 33 tuned to twice the frequency of generator H, a bias voltage across conductors 3| at twice the frequency of generator H and at least approximately in phase with the second harmonic voltage across element l4. Thus, across conductors 34 will appear a direct voltage proportional to the second harmonic voltage drop across heating element [4. This voltage, preferably but not necessarily filtered through a lowpass filter 35, which rejects preferably all but the direct component of the voltage across conductors as, is applied to an indicator which may be a meter 35, an oscillograph element, or other means of detecting current flow. This circuit then will respond to the second harmonic voltage generated across the heating element I4 due to 7 flammable materials above a minimum combustion temperature of the combustible gas.

The physical form of such apparatus as that shown in Figure 2 orits obvious equivalents de- 6-: 29a the be u For ,e

aa l iect h sam le tr nsp ted l laai T I I there. The results of the operaubnsara'shbwii on the face of meter 41, which is valen .i tmeterfii oi Fi ure If the combustibJe gas indicator isv to be used is obvious that a different form of case wouldbe used and a motor-driven gas pump would be employed instead of the bulb 42. As has been stated, the equipment which has been described in detail and the method of employment of such types of equipment has been by Way of illustration. The scope of the invention is best defined by the scope of the appended claims.

I claim:

1. In combustible-gas detectors, the combination comprising a heating element, means for exposing said element to a combustible gas to be detected, an alternating-current generator connected to said element and cyclically heating said element through a, temperature range from below to above the minimum combustion temperature of said gas, whereby the resistance of said element changes during each cycle, and a harmonic analyzer electric detector responsive to variations in voltage drop of said element only at a multiple of the frequency of said generator, said detector producing an indication upon heating of said element by local combustion of said 0 gas.

2. In combustiblegas detectors, th combination comprising a heating element, means for exposing said element to a combustible gas to be detected, an alternating-current generator and a direct-current source both passing current through said element whereby the temperature of said element varies each cycle through a temperature range including the minimum combustion temperature of said gas, and electric harmonic analyzer means for indicating a quantity proportional to an even harmonic of the voltage variation produced in said element by said alternating-current generator.

3. In combustible-gas detectors, the combination comprising a heating element, means for exposing said element to a combustible gas to be detected, a low-frequency alternating-current generator and a direct-current source both passing current through said element with peak alternating current less than the direct current supplied to said element whereby the temperature of said element varies each cycle through a temperature range including the minimum combustion temperature of said gas, and a harmonic analyzer indicator circuit the input to which is coupled to said element and the output of which produces an indication proportional to the voltage across said element of frequency equal to the second harmonic of said generator.

4. In combustible-gas detectors, the combination comprising a heating element, means for exposing said element to a combustible gas to be detected, a low-frequency alternating-current generator and a direct-current source both passing current through said element with the peak alternating current less than the direct current supplied to said lement whereby the temperature of said element varies each cycle through a temperature range including the minimum combustion temperature of said gas, and an indicator 15 circuit including a double balanced modulator the input to which is coupled to said element and the output of which coupled to a low frequency current indicator,'said modulator being supplied with modulator voltage at a frequency equal to the second harmonic of said generator.

RALPH E. HARTLINE.

REFERENCES CITED The following references are of record in the file of this patent:

8 uN'irED STATES PATENTS Number Name Date 2,219,391 Jacobson Oct. 29, 1940 2,219,540 Miller Oct. 29, 1940 2,335,032 Sullivan NOV. 23, 1943 2,369,811 Stuart Feb. 20, 194 2,412,827 Morgan et a1 Dec. 17, 1946 2,521,130 Scherbatskoy Sept. 1950 FOREIGN PATENTS Number Country Date 279, 1'78 Great Britain Feb. 20, 1929 OTHER REFERENCES Carruthers, R. 8., Copper Oxide Modulators in Carrier Telephone Systems, published in Bell System Technical Journal, vol. 18, pages 315-337 (1939).

Lawrence, R. R., Principles of Alternatin Currents, McGraw-Hill Book 00., pages 72-79 (1922). 

1. IN COMBUSTIBLE-GAS DETECTORS, THE COMBINATION COMPRISING A HEATING ELEMENT, MEANS FOR EXPOSING SAID ELEMENT TO A COMBUSTIBLE GAS TO BE DETECTED, AN ALTERNATING-CURRENT GENERATOR CONNECTED TO SAID ELEMENT AND CYCLICALLY HEATING SAID ELEMENT THROUGH A TEMPERATURE RANGE FROM BELOW TO ABOVE THE MINIMUM COMBUSTION TEMPERATURE OF SAID GAS, WHEREBY THE RESISTANCE OF SAID ELEMENT CHANGES DURING EACH CYCLE, AND A HARMONIC ANALYZER ELECTRIC DETECTOR RESPONSIVE TO VARIATIONS IN VOLTAGE DROP OF SAID ELEMENT ONLY AT A MULTIPLE OF THE FREQUENCY OF SAID GENERATOR. SAID DETECTOR PRODUCING AN INDICATION UPON HEATING OF SAID ELEMENT BY LOCAL COMBUSTION OF SAID GAS. 